Dendritic cell (DC) administration to CD8alpha knock-out (CD8alphaKO) mice results in a strong antigen-non-specific protection to a B16 murine melanoma tumor challenge. This response is mediated by lytic NK cells and cytokine-producing CD4 cells. We aimed to determine the signals that guide tumor targeting of this response. CD8alphaKO mice in the C57BL/6 background received subcutaneous (s.c.) injections of immature DC. Mice were challenged in vivo or assayed for lytic activity in vitro to a panel of syngeneic tumors with different levels of MHC class I expression. These studies support the following conclusions: (1) DC administration to CD8alphaKO mice results in protective in vivo responses to syngeneic tumors from epithelial, neuroectodermal and hematopoietic origin; in vivo protection is independent of the level of MHC classes I and II expression. (2) The in vitro lytic activity of DC-activated NK cells from CD8alphaKO mice has sensitive and insensitive targets, which is independent of the cell lineage or the level of inhibitory self-MHC surface molecules. (3) In sensitive targets a putative activating NK ligand in DC-stimulated NK cells from CD8alphaKO mice signals directly to PI3-K, but is distinct from NKG2D.